MICROTITRATION OF VITAMIN C

Vitamin C is an essential, water-soluble vitamin. It cannot be synthesized by our bodies and must be ingested regularly. The daily requirement for vitamin C is 60 mg. Some fruits and vegetables are better sources of vitamin C than others. That is, some foods have more vitamin C for each gram of food than other foods. We will try to determine which foods have more vitamin C per gram.

Purposes:

• To study redox titration.
• To study dilution calculations and the concept of concentration.
• To determine mass of vitamin C in a diluted sample solution (mg/mL).
• To determine the mass of vitamin C in an aqueous extract from a measured mass of food (mg/mL).
• To determine the mass of vitamin C in 100 grams of a food (mg/100g).
• To deterimine the effect of heat on vitamin C concentration in foods.
• To show that the conversion factor method is consistent with solution stoichiometry

• Obtaining a conversion factor by titrating solutions with known masses of vitamin C with a solution of DCPIP.

Although the biological role of vitamin C is not clearly understood, it is known as a coenzyme and as an antioxidant. It can prevent the oxidation of other compounds by being oxidized instead. We will use the oxidation of vitamin C to determine the mass of vitamin C in a known mass of food. The amount or concentration of vitamin C will be reported as milligrams of vitamin C per 100 grams of food. A dye, 2,6-dichlorophenolindolphenol, produces a redish solution when added to acidified water. When the dye is reduced, the solution becomes colorless. This dye reacts with vitamin C in a one-to-one ratio as shown by the redox reaction below.

A solution of dye added to a solution of vitamin C will be colorless until the equivalence point is reached. At the equivalence point any addition of dye will cause the solution to be red. We will investigate the conversion factor method for determining the mass of vitamin C in the titrated solution.

Conversion Factor: This method involves the use of a standard solution of vitamin C (2500mg/L). The standard vitamin C solution will be diluted by one-hundred times. A measured volume of the dilution will be titrated with the DCPIP solution. The titration will be repeated three times to obtain three conversion factors of mg of VitC per mL of DCPIP solution. The three conversion factors will be averaged. Vitamin C will then be obtained from various sources of foods (ones that you bring, bring at least two)and titrated with the dye solution. The vitamin C will be extracted from the foods with aqueous solutions. The extracts will be diluted a sample of each dilution will be titrated. Using the titration data and the conversion factor, the mass of vitamin C per volume of dilution can be determined. Using the dilution data, the mass of vitamin C per volume of extract can be determined. Using the extract data, the mass of vitamin C per mass of food can be determined.

Solution Stoichiometry: The conversion factor method can be checked by applyling solution stoichiometry to the titration. The sodium salt of the indicator is used for the experiment. C₁₂H₆Cl₂NO₂Na has a molar mass of 290.1 and a 0.025% solution contains 0.025 grams of the salt for every 100. milliliters of solution.

Standardizing Dye Solution:
a) Pipet 1.00 mL of a 2500 mg per liter vitamin C solution into a 100 mL graduated cylinder. Add about 10 mL of 3% metaphosphoric acid and dilute to a final volume of about 100 mL. Record the actual volume. Label and save this solution in a beaker. Obtain about 25 mL of 0.025% DCPIP solution in a dry and clean beaker.
b) Pipet about 5 mL of the diluted vitamin C solution into an Erlenmeyer flask and add a little water. Record the volume of diluted vitamin C solution used (before you add the little water).
c) Titrate this solution with the dye solution. Measure the minimum volume of dye solution needed so that the vitamin C solution remains red.
d) Repeat this titration two times for a total of three titrations.

Juice Samples:
a) Add about 2 mL of the juice to a 100 mL graduated cylinder. Add about 10 mL of 3% metaphosphoric acid and dilute with enough distilled water to obtain a final volume of about 100 mL. Record the volume.
b) Titrate about 10 mL of this sample (the diluted juice) with the dye solution. Record the amount of sample used.
c) Repeat the titration two times for a total of three titrations.

Solid Samples:
a) Weigh out about 2 grams of the solid food. Record the mass of food. Grind it with a mortar and pestle with about 10 mL of 3% metaphosphoric acid to extract the vitamin C. Decant (pour off) the liquid into a graduated cylinder and rinse the pulp with small portions of water. Add the rinse water to the graduated cylinder until about 50 mL of extract is obtained. Record the final volume of extract.
b) Titrate about 10 mL of the extract (the diluted juice) with the dye solution. Record the actual volume of extract titrated.
c)Repeat the titration two times for a total of three titrations.

Heated Samples:
Take a measured portion of the diluted sample and boil, covered, for ten minutes. Be sure that the solution does not lose all of its water. Replace any lost water and titrate as above.

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